The invention relates generally to grid-tied renewable energy systems, and more specifically to a home power supply system and associated interface for use with a grid-tied renewable energy system that intelligently controls power flow from a renewable energy source to a load through two parallel paths.
Renewable energy sources, including photovoltaic (PV) and wind turbine systems for example, are utilized with increasingly regularity by home owners as a means to generate electrical power for personal use. A majority of residential renewable energy systems are in the form of grid-tied systems that act as a supplemental power source that is used in combination with power from the utility grid, with some estimates indicating that over 300,000 such grid-tied residential photovoltaic systems are in use in the United States. Such renewable energy sources can reduce the utility grid demand or load during periods of the day when renewable energy is available by replacing a portion of the utility energy with the renewable solar and wind power. Conventional grid-tied renewable energy systems typically use a net-metering device to determine the utility energy use by measurement and integration of the instantaneous load power, minus the renewable power that is being generated. When the integrated load power exceeds the integrated renewable power, the meter indicates a positive amount of energy supplied from the grid to the customer, although a lower amount of grid energy is supplied than if no renewable power was utilized. However when the integrated renewable power is greater than the integrated load power, the meter indicates a negative amount of energy and the customer is able to return or sell energy back to the utility.
With regard to existing systems and methods for net-metering grid-tied renewable energy systems, there are several recognized drawbacks that limit the usefulness and cost effectiveness of the renewable energy system. The first drawback to today's net-metering systems is that, often times, energy is sold back to the utility by the home or business owner at a wholesale rate ($/kWh) that is less than a retail rate ($/kWh) at which the home or business owner buys energy from the utility to supply the required loads, with the owner sometimes also being subject to additional demand charges for energy bought from the utility. That is, energy is often sold back to the utility at a low wholesale rate during portions of the day when renewable energy production exceeds load energy requirements, and energy is bought from the utility at a high retail rate later in the day when renewable energy production is less (e.g., such as with photovoltaic power not being generated during evening hours) than load energy requirements, with additional demand charges also often being applied during periods of the evening when demand is high. Accordingly,
The second drawback to today's net-metering systems and methods is that, during utility power outages, the conventional net-meter interface equipment disables the renewable energy transfer both back to the utility (to protect utility personnel who might be working on utility lines to fix the grid) and also disables the supply of energy to the load, even when renewable energy is available. This disabling of the supply of energy to the load from the renewable energy system can be particularly problematic for individuals who require a source of power (from a combination of the utility grid and the renewable energy system) to deal with their medical equipment/conditions or other critical loads, including lighting and communication equipment.
To address both the cost and availability issues associated with net-metering of grid-tied renewable energy systems, a common solution is to add a large and costly stationary energy storage device, often referred to as a “home” battery. The home battery is typically sized to provide adequate storage for the renewable energy that is intermittently generated by the renewable source, such that the renewable energy is available for use/reuse during the entire period of the day or days when no renewable energy is available. By providing such storage of the renewable energy, the home battery offers potential savings to a customer by reducing the amount of energy that needs to be purchased from the utility at high retail rate, such as during periods (e.g., evening hours) when energy production is less and energy demand is high. It is recognized, however, that the cost savings achieved via the home battery based on the mismatch of the utility $/kWh rates between selling and buying the electrical grid energy at wholesale and retail rates must be traded off versus the additional cost of the large home battery and associated power electronics and controls.
The home battery also addresses the issue of the conventional net-meter interface equipment disabling the supply of energy from the renewable source to the load when the utility grid fails, as renewable energy stored in the home battery can be used to power loads during a grid failure. However, during extended grid outages, the home battery is often not large enough to provide energy to loads for a long period of time while still maintaining an adequate state-of-charge, such that a gasoline, diesel, natural gas, propane, or hydrogen fuel-fired stand-alone emergency generator and associated battery charger or a fuel cell generator that is fueled by hydrogen, methanol, or methane may be required to maintain the state-of-charge of the home battery, adding further cost to the customer.
An additional drawback to existing arrangements of a renewable energy source, net-metering device and home battery is that the overall system efficiency of the system from the renewable energy source to the customer load is not optimal. That is, efficiency of the overall renewable energy system is reduced due to the round-trip efficiency losses—which include conversion and battery efficiency losses associated with initially transferring the renewable energy into the home battery and conversion and battery efficiency losses associated with later transferring the energy from the battery to the customer load(s).
It would therefore be desirable to have a system and method for intelligently controlling power flow from a renewable energy source to a load and energy storage device in a grid-tied renewable energy system that overcomes existing cost and availability issues associated with net-metering of grid-tied renewable energy systems. It would also be desirable for such a system and method to provide an improved efficiency in transferring power from the renewable energy source to the customer load that minimizes losses. It would be still further desirable for such a system and method to be retrofittable with existing grid-tied renewable energy systems.